This invention relates to a bit holder for a replacement bit of the cutting head of a mining machine.
Because of the forces on a bit of a cutting head during mining and the abrasiveness of the contact of the bit with the mined material, e.g. coal, bits are subject to high wear and must be replaced at periodic intervals. There have therefore been numerous developments both in the design of the bit to achieve low cost and reliability and in the design of the bit holder to achieve a secure fixing of the bit which permits easy bit replacement.
A form of bit in common use is shown in FIGS. 1 and 2. The bit comprises a conical head member with a tungsten carbide tip 4 and a square section shoulder 6. A shaft portion 8 is provided and a groove 10 at the end of the shaft receives a retaining spring 11. Shaft 8 is insertable in a bore 12 of a bit holder 14. The shaft 8 extends through the bore 12 so that the groove 10 projects from the rear of the bore 12 whereby retaining spring 11 can be affixed to the groove by means of pliers in order to clamp the bit to the bit holder. When clamped the rear surface of right angled shoulder 6 abuts against the front face 16 of the holder in order to transfer the axial loading of the bit to the bit holder. Such a bit has numerous advantages, namely is simple construction, the effective way in which forces are transferred to the bit holder, and the fact that in use, the bit rotates in the holder so that the bit is evenly worn around its tip.
There are various wellknown modifications to the above type of bit. For example the square section shoulder of the conical head may be replaced with a tapering shoulder. The shoulder may be dispensed with altogether in which case the bit is of pencil form and is secured by its end resting on an anvil portion of the bit holder. In cases where the bit is secured in a bit holder so that its end is not accessible, bits have been designed with a resilient "wedding band" around the shaft of the bit which is compressible so that the shaft of the bit can be inserted into a bore of the bit holder, the wedding band being thereafter expandable into an enlargement of the bore to retain the bit in the bore.
The present invention is particularly concerned with bits of this type, namely bits having a pointed head portion axially extending from a shaft portion.
As regards the form of the bit holder, the design considerations must take account of two conflicting requirements namely the problem of wear of the bit holder and the problem of secure fixing of the holder to the cutting head. In addition the bit must be easily replaceable in the holder.
One common form of bit holder construction is to provide a two part holder having a base part and a holder part. The base part is securely attached to the cutting head, such as for example by welding the base part to a rotating drum of the cutting head. The base part is arranged releasably to secure the holder part so that the holder part can be replaced simply if it subject to wear while the base portion which is not usually exposed to the same amount of wear can be left in position. Problems have arisen with bit holders of this two part construction in releasably securing the holder part to the base part. One approach has been to provide a base part formed as two opposing side members defining therebetween a channel into which fits an elongate depending lug of the holder part. A pin releasably secures the elongate lug in the channel. However movement of the lug in the channel has provided to be excessive and has caused unnecessary wear of the holder part and base part.
To meet this problem the bit holder as shown in FIGS. 1 and 2 was devised. This bit holder is the subject of Australian patent application Nos. 50887/79, 12509/83 and 12510/83 and has two parts, a holder part 14 and a base part 30. The holder part has transversely projecting leg sections 18 projecting to the side of the holder part having upper and lower reaction surfaces 20a and 20b. A locating shank portion 22 is provided projecting directly beneath the bit 2. Shank portion 22 has vertically extending front and rear reaction surfaces 24,26. A forward extension portion 28 is provided projecting forwardly beneath the bit.
The holder part 14 is mounted in a base part 30. The base part is of generally rectangular shape but has side walls cut away to provide reaction surfaces 32a and reaction surfaces 32b generally at right angles to surfaces 32a. These reaction surfaces 32a and 32b co-operate with the reaction surfaces 20a and 20b of leg members 18 of the holder part. In addition the rear wall of base part 30 has a vertically extending flat surface 34 for co-operating with rear vertical reaction surface 26 of shank portion 22. In order to locate and lock the holder part into the base part, a set screw 36 and jam nut 38 are provided for threading into a bore 40 in the front face of base part 30. The end of set screw 36 abutts against the vertical front face 24 of shank portion 22 of the holder part. This has the effect of jamming the reaction surfaces 30a, 30b and 26 of the holder part against the reaction surfaces 32a, 32b and 34 of the base part. Sideways twisting movement is prevented by forward extension portion 28 engaging a mating groove 42 in the top surface of the base part 30.
In use, the reaction surfaces 20a, 20b of the transverse legs and shank portion 26 counteract the force developed in bit 2 by forward motion of the cutting head. Forces arising in the vertical direction perpendicular to the direction of forward motion are accomodated by lower reaction surfaces 20b of legs 18 and by extension portion 28 engaging in mating groove 42. Extension portion 28 also serves to prevent sideways twisting movement arising during cutting. Now the forces arising from forward motion of the cutting head produce a couple between the head of the bit and the reaction surfaces 20a and 20b. This couple which would tend to prise holder part 14 out of its mounting is resisted by set screw 36 abutting against reaction surface 24. In addition, a couple is created between reaction surfaces 20a, 20b and the head of bit 2 by vertical forces acting on the head of the bit. This couple is resisted mainly by extension portion 28.
The above bit holder of the type defined is popular on account of its solidity and resistance to movement. The bit holder is normally used in a continuous miner for board and pillar coal mining where the cutting head is normally formed as a rotating drum having its axis of rotation parallel to the coal face so that bits on the cylindrical surface of the drum cut into the coal face as the cutting head is moved up and down the coal face. The bit is suitable for use in such situations since the length of the bit is relatively small being about 6 inches from tip to base and the amount by which the bit advances while it is actually in contact with the coal face is relatively little. However the bit shown in FIGS. 1 and 2 is not as suitable for use in longwall mining applications. In longwall mining applications a cutting head is commonly provided comprising a drum rotating in a plane parallel to the coal face. Bits around the cylindrical surface of the drum cut by shearing into the coal face. The length of the bit in a longwall cutting head is relatively long being about 8 inches from tip to base. In addition the movement of the bit whilst in contact with the coal face is relatively large. This requires a bit having large clearances therearound in order not to create coal dust. It can be seen that the bit as shown in FIGS. 1 and 2 does not have large clearances in particular it has little rake clearance, by which is meant the clearance vertically beneath the tip and indicated by the region A in FIG. 1. It can be seen this region is small on account of the surfaces of the holder part extension portion 28, and the mounting of set screw 36. However it is necessary to have portion 28 and set screw 36 in these regions, among other reasons, to resist the couples referred to above created in use.
The common type of bit for longwall mining applications is shown in FIG. 3 and is configured in the general shape of an axe 50 with the axe cutting edge 52 being disposed in the forward direction of motion of the cutting head of the mining machine. The shank of the axe-like bit is secured in a holder 54 by a locking device (not shown). This type of bit has a wide rake clearance, i.e. the area beneath edge 52 and therefore does not create excessive coal dust. However a disadvantage of this type of bit is that on account of the more intricate shape it is far more expensive than the simple pointed shaped bit shown in FIG. 1, about 10 times more as expensive.
A bit holder of the two part type for use with a bit having a pointed head extending axially from a shaft has been used in longwall mining applications, and is shown in FIG. 4.
The bit holder comprises a holder part 60 having a part conical sleeve 62 having a bore 64 for receiving the bit 66. Bore 64 has a recess 68 for receiving a wedding band 70 of the bit. The sleeve is matingly engaged in a conical bore 72 of a base part 74, and the end 76 of the part conical sleeve extends through the end of the conical bore as a threaded bolt portion. A nut 78 secured on the bolt portion secures the sleeve within the conical bore. A plunger 80 extends from bit 66 and extends through bolt portion 76 and is provided for removing the bit from the holder by longitudinal movement of the plunger within the bore. An advantage of this arrangement is that it does have a relatively large rake clearance beneath bit 66. However a disadvantage is that the bit holder can only be used with the "wedding band" form of bit, referred to above, since the base end of the bit cannot project from the end of the bit holder in view of the length of the bit holder. The holder requires a plunger in order to dislodge the bit and this increases the expense of the bit holder.
This invention is particularly concerned with bit holders of the two part type having a base part for securing to the cutting head of a mining machine, and a holder part releasably secured to the base part and holding a bit, the bit being of the type having a pointed head portion axially extending from a shaft portion.
It is an object of the present invention to provide a bit holder of the two part type which can accept any form of securing of the bit in the holder but which provides a large rake clearance such as to permit the operation in longwall mining without the creation of excessive dust.
The present invention provides in one specific aspect a bit holder for a replaceable bit having a pointed head portion axially extending from a shaft portion, the bit holder having a holder part for receiving the bit and having a base part for securing to a cutting head, and means releasably securing the holder part to the base part, the holder part having at least one reaction surface co-operating with at least one reaction surface of the base part for counteracting forces arising on the bit in use of the cutting head, and said releasable securing means comprising a bolt means for extending from one of the holder part or base part into a co-operating aperture in the other of the holder part and base part for securement therein whereby to grip the holder part and the base part together with the reaction surfaces therebetween, and the bolt means being positioned relative to the reaction surfaces and the position of the bit so that the gripping force of the bolt means counteracts a couple created in operation between the head of the bit and the reaction surfaces.
In accordance with the invention by arranging that the gripping force of the bolt means should counteract the couple created in operation between the head of the bit and the reaction surfaces a very compact configuration of the holder part and base part can be arranged, with a large rake clearance. This is because in contrast with the arrangement shown in FIGS. 1 and 2 it is not necessary to have a set screw and front portions of the holder and base parts extending directly beneath the head of the bit in order to counteract the couples created during cutting. The bolt means may be integral with the base part or the holder part and extend into an aperture in the other of the base part or holder part where it is secured by means of a nut on the end of the bolt. Alternatively the bolt means may be separate from the holder part and base part and extend through registering apertures in the base part and holder part. It may either be secured by a nut or one of the apertures may be threaded to receive the bolt.
In order to ensure a compact arrangement of the bit holder and to provide clearly defined reaction surfaces which rest against one another properly, the reaction surfaces are reduced to a minimum, both in size and number, so that they remain adequate to counteract the stresses arising in use but are not so large as to increase the bulk of the holder. As preferred two sets of generally orthogonal reaction surfaces are provided for absorbing cutting forces in perpendicular directions. One set of reaction surfaces is for convenience disposed in a "vertical" plane by which is meant a lane perpendicular to the forward direction of movement in order to counteract forces arising in the forward direction of movement. The other set of reaction surfaces is disposed in a "horizontal" plane at right angles to the vertical plane. This lends itself to a particularly simple shape of holder part, comprising a shank portion extending from a sleeve portion the sleeve portion receiving the bit, the shank portion having a vertical side abutting the base part and containing vertical reaction faces and a horizontal bottom surface abutting the base part containing horizontal reaction faces. With such a simple shape of shank, it is easy and within manufacturing tolerances to provide a close mating of the holder part with the base part. This is necessary in order to permit the bolt apertures in the holder part and base part to register with one another across the interface between the two parts.
The vertical side having vertical reaction surfaces is preferably cut away to provide clearly defined upper and lower reaction surfaces, the upper reaction surface being disposed close to the sleeve portion so as to reduce stresses arising from the distance between the reaction surfaces and the bit tip. A lower reaction surface is provided adjacent the bolt apertures so that the bolt when secured grips the holder part and base part together across the lower reaction surfaces. The distance between the upper and lower reaction surfaces is such that the gripping force exerts a moment about the upper reaction surfaces which is sufficient to counteract the couple exerted between forward cutting forces exerted on the bit tip and the upper reaction surfaces. Since the upper reaction surfaces are disposed a relatively large distance from the bit tip, the lower reaction surfaces are likewise disposed a relatively large distance from the upper reaction surfaces. Thus the total distance between the cutting tip and lower reaction surfaces is relatively large and makes the holder suitable for longwall mining applications where the total height of the cutting tip above the base of the holder is about 8 inches.
The horizontal reaction surfaces disposed at the base of the shank of the holder part serves to counteract vertical forces arising on the bit tip. There is a couple created between the horizontal reaction surfaces and the bit tip. As preferred this is counteracted by a further bolt means extending between registering apertures in the holder part and bore part and disposed in such a position in relation to the horizontal reaction surfaces that the moment exerted between the further bolt means and horizontal reaction surfaces counteracts the couple.
As preferred the shank portion is groove shaped in section, with the groove disposed in the vertical side of the shank defining the reaction surfaces. The groove matingly engages a vertically disposed rib in the base part which provides resistance against sideways twisting forces arising during cutting. The upper and lower vertical reaction surfaces are defined by the free edges of the groove, and the lowermost horizontal reaction surfaces are defined by the end faces of the groove. An an alternative arrangement, the shank portion of the holder part may be T-shaped in cross-section and the co-operating base part may be groove shaped in cross-section so that the reaction surfaces are defined by the inner walls of the bar of the T.
The above arrangement is suitable for longwall mining applications since it provides a large rake clearance and provides the necessary height of the cutting tip over the base. However the bit holder according to the invention may be adapted for use with continuous miner applications, where it is not necessary for the tip to be at such a height relative to the base of the holder. In accordance with the invention, the main reacting surfaces for absorbing forces arising from forward motion of the cutting head which are in the above arrangement disposed beneath the sleeve of the holder part are in accordance with an embodiment for continuous mining provided on each side of the sleeve portion of the holder part. These reacting surfaces are preferably vertical and co-operate with vertical surfaces of the base part. The remainder of the holder part extends as a shank portion vertically beneath the upper vertical reacting surfaces to provide lower reacting surfaces adjacent registering bolt apertures for receiving the bolt for securing together the base part and the holder part. Thus the gripping force exerted on the lower reacting surface exerts a moment about the upper reacting surface which serves to counteract the couple which is created in use between the cutting tip and the upper reacting surface. In this embodiment the lowermost part of the holder part has a generally horizontal reacting surface for absorbing forces perpendicular to the direction of motion of the cutting head. In contrast to the first embodiment only a single bolt means is provided and this is positioned to exert a moment about the lowermost horizontal reacting surface in order to counteract the couple between the lowermost reacting surface and the bit tip in order to provide a secure mounting of the holder part to the base part. Although in this continuous mining embodiment the reacting surface portions of the holder part are raised relative to the sleeve part so as to make a more compact holder part there is nevertheless a wide rake clearance beneath the cutting tip since the bolt means are positioned beneath the upstanding sleeve. Thus compared with the arrangement shown in FIGS. 1 and 2 the bit holder provides a wide rake clearance which creates far less dust in board and pillar mining.
In a further aspect, the invention provides a bit holder for a replaceable bit having a pointed head portion axially extending from a shaft portion, the bit holder having a holder part for receiving the bit, and the bit holder having a base part for securing to the cutting head, and means releasably securing the holder part to the base part, wherein the holder part comprises a sleeve portion for receiving the bit and a shank portion extending beneath the sleeve portion the shank portion having a longitudinally extending side and an end engaging with the base portion and defining reaction surfaces disposed at an angle to one another for counteracting forces arising during operation of the cutting head.
In a further embodiment, the reacting surfaces of the holder part are provided as a generally part conical portion extending beneath a sleeve part for receiving the bit. The part conical portion extends into a part conically shaped bore in the base part and matingly engages in this part conical bore. A long bolt means extends from the rear end of the part conical bore and extends through a central bore in the conical portion of the holder part in order to securely engage the part conical part within the conical bore. Since the long bolt means can extend the whole length of the part conical part, a sufficient torque can be exerted for overcoming dimensional mismatching between the part conical part and the vertical bore to ensure a better seating than the known type of bit holder employing mating conical surfaces described above. The mating conical surfaces provide reaction surfaces which serve to counteract all the forces arising on the bit during a cutting operation. However if desired a locating lug may be provided vertically beneath the upstanding sleeve part for locating the holder part in a vertical direction.
Thus in a further aspect the present invention provides a bit holder for a replaceable bit having a pointed head portion axially extending from a shaft portion, the bit holder having a holder part with a bore for receiving the bit and a base part for securing to a cutting head, and means releasably securing the holder part to the base part, the holder part having a sleeve portion for receiving the bit laterally displaced from a reaction surface portion extending generally beneath the upstanding sleeve part, the reaction surface portion being part conical in shape and being matingly engaged in a part conical bore of the base part, and said releasable securing means comprising a bolt means extending through the part conical bore and a registering cylindrical bore in the part conical portion of the holder part.
In accordance with this further aspect of the invention, it is possible to provide a wide rake clearance for the bit holder since the conical surfaces may be mounted beneath and behind the upstanding sleeve part.